Gravity fed Shower mixer ; cold from mains, hot from cylinder tank

[Ghazali]

Hi, posted this message on another thread, but didn't quite get any definitive answers. Please consider:

I am thinking of running both the basin mixer and bath shower/mixer cold taps off of the mains, whilst still having the hot from the hot cylinder. (Hot will not be pumped).

My thinking is that I will turn up the thermostat on the hot cylinder so that it gives much hotter temperature, and then I will only need to use a small amount of hot flow in the mixer shower, thus utilising mainly the increased pressure/flow of the cold mains, whilst still getting a decent temperature.

Now I know it is not the done thing to do this, but before giving your comments please consider the following:

1. A kithchen sink mixer also usually runs cold off the mains and hot from the storage cylinder, yet I never hear pple complaining about 'pressure imbalance' for washing up purposes. Why is it any different for a shower.

1. I usually have an early morning shower, at a time when no-one else is using any other water supply in the house, so there is no risk of any sudden temperature/flow change from, say, another tap being used elsewhere.

2. Because of the usual daily timing of the shower, surely the mains pressure will also be similar. (I have heard it can vary depending on how many other pple are using it on the street, or at different times of the day).

1. If the water is coming through a shower mixer (i.e. pre-mixed), why would a pressure imbalance between hot and cold have any noticeable effect?

BTW: I'm not a plumber remember - so please could any comments/suggestions be of a sort that I can understand and relay to my plumber in layman's terms. Cheers

[Chris J Dixon]

A kitchen mixer doesn't generally combine the two flows within the body of the tap, it is only after leaving the nozzle that they mix. They do not, therefore, get any chance to interact, whatever their relative pressures.

The mixer will be trying to achieve the set temperature by opening and closing apertures for the two flows, If they are at significantly different pressures it will be difficult to limit the higher pressure flow to admit enough of the low pressure one.

Chris

[Christian McArdle]

This won't work. Assuming the shower mixer is pressure balancing with sufficient range, it will throttle off the cold mains to match the crap hot pressure, giving you a pathetic shower. If it isn't pressure balancing (i.e. manual valve), you'll get cold out of the shower head and the cold mains will push back up through the hot water cylinder and dump its contents through your overflow.

Your choices:

1. Use a venturi type shower mixer.
2. Pump the hot
3. Add gravity cold supply and pump hot and cold.
4. Install mains pressure hot water.

Options 1 & 2 will provide good results at minimum cost. 3 is really not worth it. Lots of work for no real advantage over 2. 4 would be much more expensive still, involving the replacement of either your boiler or hot water cylinder and should only really be considered if either need replacement already.

Christian.

[Ghazali]

Right, so the if higher pressure will be the cold, that stops me getting scalded, which is what I'm most worried about.

If there is a concern that not enough of the lower pressure flow (in this case the hot) will be admitted surely this will be sorted (admittedly with a bit of initial fine tuning) by setting a high enough temperature for the hot on the cylinder thermostat.

So far, this is sounding like a good idea......Would someone please provide a convincing argument as to WHY I DEFINITELY should NOT try this !

It sounds like a win win situation, (Especially as under my current arrangement (cold fed from storage tank in loft) the hot cylinder thermostat is set to its minimum setting, and even then I only need to set the mixer flow very slightly to hot to get a comfortable temperature).

[Christian McArdle]

The valve will reduce the pressure to that of the hot water supply. You will get a drizzle out of the shower head. It will be utterly pathetic and no better than a normal gravity shower, possibly with only about 2 feet head, if the shower is on the floor below the tank. If you don't have a pressure balancing valve, the shower will run cold and your hot water will leave the building via the header tank overflow.

Christian.

[IMM]

This sia lose, lose, situation. The pressure at the shower mixer should be about equal,otherwise the high pressure cold may work its way bacj up the not pipe into the cylinder. temperature adjustmnent is applaing giving scalds.

Best use on the basin hot from cylidner and cold from loft tank with a low pressure mixer; not for mains pressure. On the shower use a venturi, or somethimes called hydro, shower mixer. This takes cold from the mains and hot from the low pressure cylidner giving a power type of shower. B&Q now sell them. Ideal Standard Trevi Boost is an up market version. The kitchen mixer? Get a mixer to suit; low hot and high cold pressure These are commonly available. The bath? Hot from the cylinder and cold from the loft tank, usings a low pressure mixer.